Fish feed at different depths as a function of food availability, water temperature, salinity and a wide variety of environmental factors. This necessitates that a successful fisherman present his bait to the fish at the depth at which they are currently feeding. This is not always possible when artificial baits are used because the artificial bait or lure which has the most appealing motion through the water for the particular species of fish being sought may not operate properly at the depth at which the fish are.
Artificial baits or fishing lures used for trolling, casting, or jigging are designed to operate at predetermined depths. The depth of operation of a fishing lure is a function of the density of the lure assembly, its hydrodynamic shape and speed of forward motion. Lure density and shape are fixed during manufacture so the depth at which it will operate properly is fixed and a user cannot vary the operating depth of a lure to place the lure at the same depth as the fish. In some cases a lure may be made to dive deeper or shallower by varying the retrieval rate but the shape of a lure dictates that it should be retrieved or trolled at a set speed to achieve the most lifelike action. Thus, a successful fisherman must have a large variety of fishing lures designed to operate at various depths at which fish may be found and in many cases a lure having the proper motion through the water does not have the proper density and therefore the lure which should be most successful in attracting fish fails to reach them.